The present disclosure relates to systems and methods that provide optical signals over a cable transmission network.
Referring to FIG. 1, cable TV (CATV) systems were initially deployed as video delivery systems. In its most basic form, the system received video signals at the cable head end, processed these for transmission, and broadcast them to homes via a tree and branch coaxial cable network. In order to deliver multiple TV channels concurrently, early CATV systems assigned 6 MHz blocks of frequency to each channel and Frequency Division Multiplexed (FDM) the channels onto the coaxial cable RF signals. Amplifiers were inserted along the path as required to boost the signal, and splitters and taps were deployed to enable the signals to reach the individual homes. Thus, all homes received the same broadcast signals.
As the reach of the systems increased, the signal distortion and operational cost associated with long chains of amplifiers became problematic, and segments of the coaxial cable were replaced with fiber optic cables to create a Hybrid Fiber Coax (HFC) network to deliver the RF broadcast content to the coaxial neighborhood transmission network. Optical nodes in the network acted as optical to electrical converters to provide the fiber-to-coax interfaces.
As the cable network evolved, broadcast digital video signals were added to the multiplexed channels. The existing 6 MHz spacing for channels was retained, but with the evolving technology, each 6 MHz block could now contain multiple programs. Up to this point, each home received the same set of signals broadcast from the head end so that the amount of spectrum required was purely a function of the total channel count in the program line-up.
The next major phase in CATV evolution was the addition of high speed data service, which is an IP packet-based service, but appears on the HFC network as another 6 MHz channel block (or given data service growth, more likely as multiple 6 MHz blocks). These blocks use FDM to share the spectrum along with video services. Unlike broadcast video, each IP stream is unique. Thus, the amount of spectrum required for data services is a function of the number of data users and the amount of content they are downloading. With the rise of the Internet video, this spectrum is growing at 50% compound annual growth rate and putting significant pressure on transmission quality parameters such as available bandwidth, signal distortion, etc. Such pressure has been further increased with the advent of narrowcast video services such as video-on-demand (VOD), which changes the broadcast video model as users can select an individual program to watch and use VCR-like controls to e.g. start, stop, and fast-forward. In this case, as with data service, each user requires an individual program stream.
FIG. 1 shows part of the cable TV infrastructure, which includes the cable head end, the Hybrid Fiber Coax (HFC) transmission network, and the home. The CATV head end receives incoming data and video signals from various sources (e.g., fiber optic links, CDN's, DBS satellites, local stations, etc.). The video signals are processed (reformatting, encryption, advertising insertion etc.) and packaged to create the program line up for local distribution. This set of video programs is combined with data services and other system management signals and prepared for transmission over the HFC to the home. All information (video, data, and management) is delivered from the head end over the HFC network to the home as RF signals. In the current practice, systems in the head end process the signals, modulate them to create independent RF signals, combine these into a single broadband multiplex, and transmit this multiplex to the home. The signals (different video channels and one or more data and management channels) are transmitted concurrently over the plant at different FDM frequencies. In the home, a cable receiver decodes the incoming signal and routes it to TV sets or computers as required.
Cable receivers, including those integrated into set-top boxes and other such devices, typically receive this information from the head end via coaxial transmission cables. The RF signal that is delivered can simultaneously provide a wide variety of content, e.g. high speed data service and up to several hundred television channels, together with ancillary data such as programming guide information, ticker feeds, score guides, etc. Through the cable receiver's output connection to the home network, the content is delivered to television sets, computers, and other devices. The head end will typically deliver CATV content to many thousands of individual households, each equipped with a compatible receiver.
Cable receivers are broadly available in many different hardware configurations. For example, an external cable receiver is often configured as a small box having one port connectable to a wall outlet delivering an RF signal, and one or more other ports connectable to appliances such as computers, televisions, and wireless routers or other network connections (e.g., 10/100/1,000 Mbps Ethernet). Other cable receivers are configured as circuit cards that may be inserted internally in a computer to similarly receive the signals from an RF wall outlet and deliver those signals to a computer, a television, or a network, etc. Still other cable receivers may be integrated into set-top boxes, which receive an input signal via an RF cable, decode the RF signal to separate it into distinct channels or frequency bands providing individual content, and provides such content to a television or other audio or audiovisual device in a manner that permits users to each select among available content using the set top box.
As previously mentioned, the CATV transmission architecture has been modified to permit data to flow in both directions, i.e. data may flow not only from the head end to the viewer, but also from the viewer to the head end. To achieve this functionality, cable operators dedicate one spectrum of frequencies to deliver forward path signals from the head end to the viewer, and another (typically much smaller) spectrum of frequencies to deliver return path signals from the viewer to the head end. The components in the cable network have been modified so that they are capable of separating the forward path signals from the return path signals, and separately amplifying the signals from each respective direction in their associated frequency range.
FIG. 2 shows a Hybrid/Fiber Coax (HFC) cable network. A head end system 120 includes multiple devices for delivery of video and data services including EdgeQAMS (EQAMs) for video, cable modem termination systems (CMTS) for data, and other processing devices for control and management. These systems are connected to multiple fiber optic cables 100 that go to various neighborhood locations that each serve a smaller community. A fiber optic neighborhood node 130 is located between each fiber optic cable 120 and a corresponding trunk cable 140, which in turn is interconnected to the homes 160 through branch networks and feeder cables 150. Because the trunk cable 140, as well as the branch networks and feeder cables 150, each propagate RF signals using coaxial cable, the nodes 130 convert the optical signals to electrical signals that can be transmitted through a coaxial medium, i.e. copper wire. Similarly, when electrical signals from the home reach the node 130 over the coaxial medium, those signals are converted to optical signals and transmitted across the fiber optic cables 100 back to the systems at the head end 120. The trunk cables 140 and/or feeder cables 150 may include amplifiers 170. Connected to each trunk cable 140 is a branch network that connects to feeder cables (or taps) that each enter individual homes to connect to a respective cable receiver. This is generally referred to as Fiber-to-the-Neighborhood (FTTN) or Fiber-to-the-Curb (FTTC), depending on how close the optical nodes are to the viewer's home.
Hybrid fiber/coax networks generally have a bandwidth of approximately 500 MHz or more. Each television channel or other distinct content item transmitted along the forward path from the head end to a user may be assigned a separate frequency band, which as noted earlier has a typical spectral width of 6 MHz. Similarly, distinct content delivered along the return path from a user to the head end may similarly be assigned a separate frequency band, such as one having a spectral width of 6 MHz. In North America, the hybrid fiber/coax networks assign the frequency spectrum between 5 MHz and 42 MHz to propagate signals along the return path, and assign the frequency spectrum between 50 MHz and 750 MHz or more to propagate signals along the forward path.
Referring to FIG. 3, a cable modem termination system (CMTS) 200 may be installed at the head end, which instructs each of the cable modems when to transmit return path signals, such as Internet protocol (IP) based signals, and which frequency bands to use for return path transmissions. The CMTS 200 demodulates the return path signals, translates them back into (IP) packets, and redirects them to a central switch 210. The central switch 210 redirects the IP packets to an IP router 220 for transmission across the Internet 230, and to the CMTS which modulates forward path signals for transmission across the hybrid fiber coax cables to the user's cable modem. The central switch 210 also sends information to, and receives information from, information servers 240 such as video servers. The central switch 210 also sends information to, and receives information from, a telephone switch 250 which is interconnected to the telephone network 260. In general, cable modems are designed to only receive from, and send signals to, the CMTS 200, and may not communicate directly with other cable modems networked through the head end.
Using this architecture, forward path signals from the head-end are broadcast to all cable modem users on the same network or sub-network. Each cable modem filters out the portion of the signal it needs, which may then be selectively provided to the user. Along the return path, each cable modem delivers a signal to the head end through the CATV network, and which occupies a part of a spectrum shared among other cable modems. Therefore, the system may regulate which modem's return path signal is delivered to the network at which time using time or frequency division multiple access (TDMA or FDMA),